1. Field of the Invention
The present invention relates in general to optical imaging, and more in particular to an optical system with a catadioptric optical subsystem which may be used as a microscope's objective system or a lithographic projection system, for example.
2. Description of Related Art
A catadioptric system, which includes a combination of catoptric (reflective) and dioptric (refractive) optical components, can be used as a microscope objective system or lithographic projection system. When an axisymmetric mirror is used in such a system, there is a light-blocking portion (i.e., an obscured portion) on the optical axis of the system. An obscuration ratio—which characterizes the fraction of blocked light—is defined by Equation (1), as follows.
                    Obscuration        =                                            sin              ⁢                                                          ⁢                              θ                l                                                    sin              ⁢                                                          ⁢                              θ                m                                              ×          100                                    (        1        )            
where θ1 is the lowest angle to achieve a required obscuration ratio (hereafter θ1 will be referred to as the “lowest obscuration angle”), and θm is the angle formed between the marginal ray that comes from the object and the normal to the surface where the marginal ray impinges (hereinafter θm is referred to as the “marginal angle”).
As it is known to persons having ordinary skill in the art, a quantitative measure of image quality is the modulation transfer function (MTF). MTF describes the ability of a lens or optical system to transfer contrast from the object to the image produced by the lens or system. In an optical microscope, the MTF is a measurement of the microscope's ability to transfer contrast from the object to the image plane at a specific resolution. Naturally, any obstruction placed in the light path of a microscope's imaging system not only will reduce image contrast, but will also produce a loss of energy in the intensity distribution of light detected at the image plane.
The MTF of a microscope can be obtained from the contrast generated by periodic lines spaced at a predetermined distance present in an object that result in sinusoidal intensities in the image. These sinusoidal intensities vary as a function of spatial frequency. For example, if an object having absorbing and transparent line pairs with a spatial period of 1 micron (spatial frequency 1000 lines per millimeter) is imaged with a high NA microscope, the individual lines would be imaged (resolved) with a moderate degree of image contrast. Decreasing the distance between the line pairs to a spatial period of 0.5 microns (spatial frequency equal to 2000 lines per millimeter) would reduce contrast in the final image and may not be resolvable, but increasing the spatial period to 2 microns (spatial frequency equal to 500 lines per millimeter) would increase image contrast and would be easily resolvable.
FIG. 1A illustrates a graph showing the manner in which obscuration ratio of an optical system affects the MTF. As it can be appreciated from FIG. 1A, the MTF, especially at low and middle spatial frequencies, can be degraded as the obscuration ratio becomes larger.
To increase MTF, there is a need to decrease the obscuration in the optical system while continuing to use the catadioptric subsystem.
Previous attempts to addressing the problem of obscuration have been made. For example, U.S. Pat. No. 5,650,877 to Phillips Jr. et al., discloses a lithographic projection system in which a catadioptric optical element having specially configured front and back surfaces projects a reduced image of a reticle onto a substrate with high NA radiation. The back (last) surface of the optical element (closest to the substrate) has a central aperture surrounded by a concave reflective portion. The front surface (opposite to the back surface), through which radiation illumination passes, has a partially reflective coating that transmits therethrough part of an incident light beam toward the concave reflecting portion of the back surface. The partially reflective coating provides partial transmission uniformly through the surface.
When the concave reflective portion returns the received light to the front surface, the partially reflective coating partially reflects and partially transmits the light returned by the concave reflective portion on a converging path through the central aperture to the substrate. According to U.S. Pat. No. 5,650,877, central obscuration may controlled by covering part of the front surface to block a part of the illumination beam corresponding to direct light that would not be reflected by the concave reflective portion. However, although relatively low obscuration may be obtained by blocking the direct light, substantial energy loss is caused by uniform partial transmission and reflection of the front surface. Japanese patent application publication JP2002-82285 also discloses similar method related to the use of a semitransparent coating.
B. S. Blaisse et al., in “Catadioptric microscope objective with concentric mirrors”, APPLIED SCIENTIFIC RESEARCH, SECTION B, Volume 2, Number 1 (1952), pages 453-466, disclose another way of using a semitransparent coating. Specifically, Blaisse discloses the use of a partial reflection coating, as follows. A semitransparent coating whose reflectance is 50% is coated only on section between points P1P2 on a convex surface, not the whole surface. 100% reflectance coating is put in section P2P2′ on the same convex surface. Light coming from object O is divided into 3 light ray groups G1 to G3 as shown in FIG. 1B. Light ray G1 is between P0 and P1 on surface 4, G2 is between P1 and P2 on surface 4, and G3 is between P2 and P3 on surface 4. Route of G1 passes through region P0-P1 of surface 4 and is reflected by region Q1-Q2 of surface 2 without any loss. After that, region P1-P2 reflects G1 with 50% loss. Light rays G2 pass through region P1-P2 with 50% loss and are reflected by section Q2-Q3. After that, G2 is reflected by P2-P3 without any loss. Light rays G3 cannot pass through region P2-P3. In other words, region P2-P3 acts as a baffle by blocking part of the light from the object O.
From the foregoing state of the art, it is seen therefore, that a partially reflective or graded coating can be used to control the level of obscuration ratio in a catadioptric optical system. However, the obscuration ratio will inevitably degrade the modulation transfer function of the optical system. Accordingly, there is a need to decrease the obscuration ratio while optimizing the modulation transfer function.